Ionic vacuum pumps



Jan. 14, 1964 GABOR 3,118,077

IONIC VACUUM PUMPS Filed Nov. 9, 1960 I 1 N K1:

Invenfor DEA/N15 6650 f) #arne s a m at 4 3,118,077 Patented Jan. 14, 1964 3,118,077 IONIC VACUUM PUMPS Dennis Gabor, London, England, assignor to National Research Development Corporation, London, England, a British corporation Filed Nov. 9, 1960, Ser. No. 68,183 Claims priority, application Great Britain Nov. 12, 1959 5 Claims. (Cl. 313-7) This invention relates to ionic vacuum pumps in which the gas atoms or molecules are ionized by electronic impact, the ions being impelled by electric fields into contact with a getter material which removes them by absorption or adsorption. r

Ionic vacuum pumps have been known for a long time in which electrons are made to oscillate between ,two electrodes at cathode potential which the electrons cannot reach, being prevented by a strong magnetic fie1d from escaping sideways. Pumps of this type have: been successfully embodied in sealed-off vacuum devices. i 'lheir disadvantage is the large weight of the permanent magnets, and also their strong magnetic stray field which prevents their application in electron-optical devices sensitive to magnetic fields, such as cathode ray tubes or electronic cameras. "K

This invention relates to an ionic vacuum pump in which the movements of the ionising electrons are governed by purely electrostatic fields in such a way that, at least for the most part, they do not land on the appropriate electrode unless they have collided with a gas molecule. I

According to the invention an ion-getter pump comprises a box or outer electrode presenting a surface of gettering material and having a rotationally symmetrical wall, said electrode being adapted to be maintained at a I negative potential to serve as an ion collector, an axial electrode lying on the axis of symmetry of said wall and adapted to be maintained at a positive potential to serve as an anode, and means comprising an electron gun for producing ionising electrons, said gun being positioned to direct an electron stream tangentially into the device, the arrangement being such that in, operation the ionising electrons are caused to travel in extended paths within the space encircled by said rotationally symmetric wall without the aid of a magnetic field. In the use of the device, the space encircled by said rotationally symmetric wall is, of course, in communication with the space to be evacuated, the outer electrode operating as the cathode and the axial electrode as the anode. By tangentially is meant in a direction tangential to any rotationally symmetric surface centered on the axis of symmetry of the outer electrode and removed from the axis, that is to say between the axial electrode and the wall.

The device is operated with a potential difference between the outer and the axial electrodes high enough for ionization and producing a generally radial electric field between the electrodes. The potential of the outer electrode is equal to or lower than the potential of the cathode in the electron gun, so that electrons fired from the gun cannot land on any point of the outer electrode. They are also prevented from landing immediately on the positive axial electrode by their momentum, which carries them past the axial electrode and makes them orbit around it, under the influence of the electric field, until they strike a gas atom or molecule, the electrons being ultimately collected at the axial electrode. Positive ions produced by the electron stream will move to the outer electrode which is made of or coated with gettering material which permanently absorbs or adsorbs at least a considenable proportion of the atoms formed by the neutralization of the ions.

The invention has particular advantages for use in association with electronic devices such as cathode ray tubes and the like. in which voltages sufficiently high for the operation of the device are available, so that no separate supply is required. The invention is of special value in cathode ray devices such as projection tubes or colour tubes, in which gas development is strong owing to highpower o eration, or to tubes containing complicated electrode structures or large metal walls which cannot be perfectly outgassed. Moreover, the invention makes it possible to embody in such devices insulating materials such as silicones for example, which could not otherwise be employed in sealed-off high vacuum devices, because of their tendency to release gas continuously.

The invention will be better understood from the following description given with reference to the accompanying drawings in which:

FIG. 1 is a plan section of an ionic vacuum pumping device according to the invention.

FIG. 2 is an axial section thereof.

,In FIG. l the outer or negative electrode is in the form of a box having a cylindrical wall 1, and end-plates 2 and 3, the latter being made of metal gauze to give access to the gas in the vacuum envelope (not shown) which surrounds the structure. The axial or positive electrode or anode is a wire or rod 4, supported on but insulated from the end-plates by insulating bushes 5. An electron gun 6 comprising a hot cathode 7, a grid 8 and anapertured anode 9 is mounted in the wall 1 withits axis directed in a tangential direction at radius R from the axis of the wall 1. The grid 8 is preferably of cathode potential and the anode 9 is at a potential high enough to draw a sufiicient electron current from the cathode, but preferably well below that of the anode 4. This choice of the potential arises from economy of power, and also from the requirement that the anode 9 shall not'much dis- :turb the rotational symmetry of the electric field inside the box.

A typical electron trajectory is indicated in FIG. 1 in dotted lines. It will be seen from this figure that the electrons (due to their momentum and the generally radial electric field between the outer and the axial electrodes) orbit around the axis in approximately cycloidal trajectories, and that the radius R of the imaginary cylindrical surface to which the axis of electron gun 6 is tangent is several times larger than the radius of the wire or rod anode 4. Let V be the potential of the apertured anode 9; at the instant of leaving the aperture the electrons have an angular momentum around the axis proportional to RV which they will preserve in their motion. Let r be the radius of the anode 4, and V its potential. If

no electron will be able to reach the wire 4, because the largest potential available in the device, which is V will not be sufficient to make the right hand side equal to the left hand side at the radius r, hence the radius r will not be accessible.

As an example, if V :l0,000 volts, V volts, electrons will not be able to land on a wire with a radius r which is less than one-tenth of the initial radius R. In practice one must make an allowance for the disturbance of the rotational symmetry of the electrostatic field by the gun. and it is preferable to make the wire radius smaller than that given by the angular momentum condition. A lower limit is set by the heating of the wire 4, by the electron current to it due to electrons which have been in collision with gas molecules. This wire is therefore preferably made of tungsten or of titanium, or in the form of a helix surrounding a gettering material, which is gradually evaporated.

The device may, of course, differ from that shown in the above described example in a number of ways. Thus if the cylir-.drical wall of the outer electrode is deep enough in the axial direction the end walls need not be provided since there will be little likelihood of electrons.

. but preferably with a central opening rather small in relation to the maximum diameter of the electrode. In

.such a case the axial electrode or anode may be supported independently of the outer electrode. The axial extent of the outer electrode and the provision or not of the end plates or flanges will depend in some measure upon the care with which the electron gun 7, 3', 9 is focussed and the divergence of electron paths from planes perpendicular to the axis of the outer electrode avoided.

The walls of the outer electrode are preferably made of solid gettering metals, such as zirconium or titanium, or they are coated by evaporation, with getters such as barium. Though barium has a gettering effect also for neutral gases, it is known that barium getters soon go inert, and become practically inactive for gas pressures below a certain minimum, which fall far short of the re quirements in cathode ray devices and the like. They become, however, active in the presence of ions. Experimentally it was found that even after long operating times the sticking factor in the device according to the invention well exceeded that is to say more than a tenth of the ions which struck the wall were permanently eliminated.

The pumping power of the device according to the invention is about 2 litres/sec. at l() mm. pressure per milliampere of ion current if every ion is permanently removed, and 200 cmfi/ sec. if the sticking ratio is 10%. At 10* mm. Hg, which is the minimum vacuum at which cathode ray devices can operate normally, these figures increase to 200 litres/sec. and 20 litres/sec. respectively. 200-litres/sec. is about as much as a gauze-covered endplate of 6 cm. diameter can transmit; hence at even lower pressures the current can be reduced to about 100 microamperes, without loss of pump efficiency. Beam currents of this order can be taken from the high tension supply of television tubes and the like without having to increase the power packs beyond their normal size.

It is a particular advantage of the invention that the positive ion bombardment of the electron gun is small, even in bad vacua, because the radial electric field will drive most ions straight to the casing, instead of to the cathode. The life of the cathode of the pump is therefore likely to exceed that of the cathode in the main elec tron gun of the vacuum tube device with which it is associated.

1 claim:

I. An ionic vacuum pumping device comprising an outer electrode having a surface of gettering material and a rotationally symmetric wall defining a space adapted to contain gas molecules, said electrode being adapted to be maintained at a negative potential to serve as anion collector, a second electrode within said space adapted to be maintained at a positive potential to serve as an anode, and means for producing ionizing electrons, said means comprising an electron gun positioned to direct an electron stream tangentially into said outer electrode, whereby the electrons are caused to travel in exte ed paths within said space without the aid of a magnetic eld.

2. An ionic vacuum pumping device comprising an outer electrode having a rotationally symmetric wall, an axially extending electrode lying on the axis of symmetry of said wall, and an electron gun positioned to direct an electron stream into said outer electrode in a direction tangential to an imaginary rotationally symmetric surface centred on said axis of symmetry and spaced from said axially extending electrode, said outer electrode having two end walls, substantially perpendicular to said axis of symmetry, one of said end walls being foraminous.

3. An ionic vacuum pumping device according to claim 1, in which said outer electrode includes a foraminous wall which affords communication with a space to he evacuated.

4. An ionic vacuum pumping device according to claim 1, in which said electron gun penetrates said rotationally symmetric wall and is positioned to direct an electron stream tangentially to an imaginary cylindrical surface centred on the axis of symmetry of said wall and nearer said said wall than to said axis.

5. An ionic pumping device comprising an outer electrode having a rotationally symmetric wall, an axially extending rod electrode lying on the axis of symmetry of said wall, and an electron gun penetrating said rotationally symmetric wall and positioned to direct an electron stream tangentially to an imaginary cylindrical surface centred on the axis of symmetry of said wall and nearer to said wall than to said axis the radius of said imaginary cylindrical surface being several times larger than the radius of said axially extending electrode.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN IONIC VACUUM PUMPING DEVICE COMPRISING AN OUTER ELECTRODE HAVING A SURFACE OF GETTERING MATERIAL AND A ROTATIONALLY SYMMETRIC WALL DEFINING A SPACE ADAPTED TO CONTAIN GAS MOLECULES, SAID ELECTRODE BEING ADAPTED TO BE MAINTAINED AT A NEGATIVE POTENTIAL TO SERVE AS AN ION COLLECTOR, A SECOND ELECTRODE WITHIN SAID SPACE ADAPTED TO BE MAINTAINED AT A POSITIVE POTENTIAL TO SERVE AS AN ANODE, AND MEANS FOR PRODUCING IONIZING ELECTRONS, SAID MEANS COMPRISING AN ELECTRON GUN POSITIONED TO DIRECT AN ELECTRON STREAM TANGENTIALLY INTO SAID OUTER ELECTRODE, WHEREBY THE ELECTRONS ARE CAUSED TO TRAVEL IN EXTENDED PATHS WITHIN SAID SPACE WITHOUT THE AID OF A MAGNETIC FIELD. 